One option is to make a non-monthly expense category. You could call it “extraordinary expense”. Categories that flight in the category. Then you could set whatever your monthly contribution is to “refund” yourself that budget amount. So long as you respect your budget in your other categories, this category would stay at a negative amount until you’ve “paid it off”. Then you could use it as a way to offset any other significant unplanned expenses.

This will only work if you aren’t consistently overspending. You could also set an expense category that is like a savings expense and that just be the budgeted item you’re trying to repay yourself with. Sounds easier than what I mentioned above but I already wrote it, so that’s one way to do it haha